GVP HD+ problems

[Tahoe]

It is normal for the power supply not giving you any reading on a multimeter. The power supply has a so called sense-line, just like the A590. This basically means it won't switch on unless the A500 is on. It is a +5v line coming from the A500 switching on the power supply (that is why some of them do not have any power switches.

The switch on top of the A500HD8+, should that be the one you are refering to is not actually a on/off switch for the drive but more a disable/enable autoboot switch.

[Tahoe]

No!! It has a special sense line which accepts 5v, don't use it's 5v output as an input.
To check if the power supply is the one to blame best bet would be to modify it to run off the A500's power and remove the hard disk. If it works it's the PSU, if it doesn't, well... Start looking further. You could also run the hard disk power from an AT power supply and leave the SCSI cable connected. Powering the drive from the A500 might just be asking a little too much, even for a 4.3A PS.

Here is the site explaning how to modify your drive