40 to 60 GB sounds sufficient.

Yes
The difference between SSD to SSD is too little to notice
But HDD to SSD for os drive sure you can notice it

Seconded.

Put all programmes & start-up app files in SSD. If you put inside HDD for sure you get HDD speed instead of SSD.

For me I only hv 40GB of space. Howver all my programmes/ start-up files all inside SSD. Only certain large files like my Other countries GPS maps, Photos & movies in my HDD, Other such as my working words/excel & etc all inside SSD. Still I'm still having 19GB of free space out of 450GB.

Did anyone face the current problem of OCZ Vertex 2?
OCZ used 25nm NAND Flash in this part.
The usable capacity (56GB (34nm) -> 51GB (25nm)) and speed are both influenced... Be careful.
http://forums.whirlpool.net.au/forum-replies.cfm?t=1644571

I don't think there's such incompatibility issue with normal HDD as there's working independently to each other.
Check cabling and try plugging the said HDD to another SATA port and see.
what about if u removed the SSD, is the HDD still hang?

arhh my bad... i tot it was desktop with internal SATA ports 
some laptop like my Inspiron does really have weak USB power supply.
it will lost whenever i put USB soundcard.

No idea bro but i would format that USB HDD or if u're using external HDD case that could be the culprit too.