Two of my externals seem to have quit working in the last couple days.
Would moving them between a mac and pc on a regular basis corrupt them???
Even my new Mac won't see the usb HD anymore. Any suggestions?

Did both fail at the same time?
Possibly swap out to new cable. Try them on another machine.
Do they spin up? Is there a power light and if so do they light?
Test the port of the computer by installing a different device.eg.printer,camera,etc.
Basically determine if the problem is the drive itself.

Both HD's power up, ports work fine with other devices. Both HDs seem to crap out the same time???

You say they power up, can you hear or feel them spinning?
Did you swap out the cables?
Does windows see the drive in " My computer"?

Edit:Sorry for double post.

I wish I had them both with me, but I am at work.
No worries on the double post.
The spin up, get recognized then dissappear???? On a pc. They don't seem to do anything on my mac other than power up when I turn them on.
The xp system says drive not recognized.