Sorry but had trouble creating a descriptive title and explanation.

Notes:
The control cct board controls the 12V so it has to be spliced before the control cct board.
I have spare HDDs so buying an SSD is not worth it when I have all the parts and time for this mod.
The picture shows the location of the additional PS which fits perfectly in that location.

Question:
I have a PVR with multi voltage output 17v, 12v, 5v power supply for the control cct board and want to add a larger HDD. The HDDs current draw is greater than the PS can provide as stamped on the board, 12v A-0.25 so I want to cut the 12v and the corresponding GND wire and splice onto a separate power supply 12v 1.5A. Even though each voltage has it's own GND wire GND is common.

Will this have any adverse effect by not using the PVRs PSs 12v and GND and connecting the additional PS 12v and GND to the cct board.

Additional question:
If it works, I can then add a cooling fan and since the 12v to the HDD is only on when the PVR is on I would like to tap onto it. The yellow wire is 12v. Would the fan cause interferece being inductive or are PC type DC fans designed to prevent interference.

 

Don't cut the GND. New PSU must be fully isolated, which it appears to be. Add new PSU to pick up AC input and connect it's GND to the existing PSU GND on PCB as near to PSU output connector as possible. This will avoid creating any ground loops. If +12v is only used for the HDD then cut 12v wire about 1" from PSU connector (to give enough length to reconnect if need be) and attach to new PSU. If it's used on other PCB then leave alone, find +12v connection on HDD cable, cut about 1" from HDD and wire HDD direct to new PSU.

Standard 12v PC fan can be connected to new PSU without any problem.

 

connect it's GND to the existing PSU GND on PCB as near to PSU output connector as possible. This will avoid creating any ground loops. If +12v is only used for the HDD then cut 12v wire about 1" from PSU connector (to give enough length to reconnect if need be) and attach to new PSU.

This is what I was thinking though you confuse me with the comment on AC "Add new PSU to pick up AC input" Thats just connecting it up. Nothing to do with the Outputs as being discussed, correct?

If it's used on other PCB then leave alone, find +12v connection on HDD cable, cut about 1" from HDD and wire HDD direct to new PSU.

That's not an option for the cct board (green one) controls the power to the HDD, 12v and 5v. In doing what you suggest it would have constant 12v power on to the HDD even when OFF less the 5V which may cause problems now or in the future and or possibly prevent the HDD from going to sleep.

Standard 12v PC fan can be connected to new PSU without any problem.

Missread the question. Power isn't the issue with new power supply.I want to connect it to the power cables that feed the HDD so when the HDD starts it starts the fan. So the question is will a fan being inductive inline with the HDD power cable cause electrical interference to the HDD or are PC fans designed to prevent electrical interference

 

Connect the new 12V psu output to the Hdd and link the grounds together, see if it works okay with the fan as well, it might be okay without connecting the grounds together.

 

Thats just connecting it up. Nothing to do with the Outputs as being discussed, correct?

Correct

That's not an option for the cct board (green one) controls the power to the HDD, 12v and 5v. In doing what you suggest it would have constant 12v power on to the HDD even when OFF less the 5V which may cause problems now or in the future and or possibly prevent the HDD from going to sleep.

Ah, OK, that makes it slightly trickier. Does the new PSU have a standby or enable input, or can be modified to have? Link?

If not, you have three options:
- A simple 12v relay, driven by the existing 12v rail, switching the AC to the new PSU, or switching the DC output
- A solid-state relay(SSR), driven by the existing 5 or 12v rail, switching the AC to the new PSU eg Omron GCM3
- A high-side power switch chip on a small circuit board eg Infineon BTS50005-1LUA or similar.https://uk.rs-online.com/web/p/power-switch-ics/7528261

Missread the question. Power isn't the issue with new power supply.I want to connect it to the power cables that feed the HDD so when the HDD starts it starts the fan. So the question is will a fan being inductive inline with the HDD power cable cause electrical interference to the HDD or are PC fans designed to prevent electrical interference

No I didnt, the 12v to a HDD is used for the motor, so adding a 12v fan in parallel with the HDD has no impact and is quite common.