Why my W 7 ultimate 64 bits recognise only my larger usb ext hdd?

Hello, I have two large USB (1Tb and 2 Tb) external drives running on my W7 Ultimate 64 bts. But when I get my old 40 Gb ext hdd or my mate's similar unit but 60 Gb (both are generic) I can't see them. Yet if I plug them on my XP laptop, or Window 7 starter notebook they install automatically with no problem!
I can see my own device in Management but it requires to initial...and it won't, as for my mate's nothing at all happens when I plug it on the usb port, but the unit's green light is on!.

Do they show up in Disk Management ? ?
Right clik My Computer...select Manage...then Disk Management....
Right tclik the drives and select Properties.
Report your findings here.

Thank you for your reply patio. As I said, my 40 gb did show and i seem to remember it needed to be initialed, but yesterday when I plugged my mate's 60 gb nothing at all happened beside the green light on his unit. He had it working on his system ok and when I put it on my notebook I got it to installed within second
I'll give another go with mine to see the property status.

maybe just my guess they are not compatible with 64bit os.. as some old hardware is.
what do you mean by generic? usually there is HDD (IDE or SATA, in this case old IDE) and controller chip for usb conversion.

Thanks vaidas3, yes this could be a good guess but how to determine if it's the reason? I just plugged in my own 40 gb unit and went to "Disk Management" to reply Patio request. Sure enough it's listed as "Disk 5, unknown and not initialized" and a window propped up "initialize Disk" Select disk Disk 5 is ticked in the white area.
Then in the grey area below "use MBR" is ticked and GPT has an explanatory note (Guide Partition Table style is not recognized by all previous Windows...
This does not apply for this as it is lot smaller that 2 gb.
And if I click OK nothing happens that is no initialisation and a message "disk not ready" shows up?
Well, Yes it seems it's not compatible with 64 bits...amongst other possiblities!
And I can't even see my mate's under Disk management?