time machine suddenly not working, advice pls

Hi,

I have two external HD's, one is mains powered and the other is a smaller and more portable USB powered one..

i used to use time machine on the more portable one, but then i changed to using the mains powered ext HD for time machine backing up, and never had a problem with either.

Now i am away from home and only took the portable one with me, and the first time i tried to back up using time machine with this one it fails always after backing up about 30gigs...it always seems to fail at the same point more or less... and the msg i got is below...

One thing to note, is that before attempting this back up, this portable ext HD did have a lot of content on it (mainly music), which is now not backed up on either of my ext HD's - i transferred it to my laptop + deleted it from the external HD before making the time machine backup. now i can't even simply copy and paste anything to the empty portable ext HD. I can remember having problems deleting from the portable ext HD and i think, i had a lot of problems specifically removing the backups folder that was on that portable HD from a long time ago, but i did remove it in the end.

So my suspicions are that this is probably the reason it doesnt work now - odd that i can't even put anything onto the portable HD now... I need to return this laptop to apple asap (due to other non-related reasons), so i need to erase its HD before sending and i cant access the bigger ext HD with my backups on,( to make a new back up) before i need to send it, (that's why I wanted to change this portable HD that's with me to be my time machine back up)as i have a recent back up at home I'd happily just put my new things on a storage device you see, as could use the back up again later when needed.

using time machine is essential for me for its simplicity of transferring everything to the new comp, (likely to be a mac mini when ive done enough research)...but as i say - because i have a fairly recent time machine back up at home, if i can simply transfer the latest additions to my laptop to this ext HD, whilst not ideal, will suffice as i don't have much time.

does anyone have any idea why this is failing? I really need to back up to this ext HD as i am a long way from my usual one and won't be able to access it for a while. Was I not able to change back up device more than once? hopefully its just a case that this portable external HD has just stopped working and a solution is to buy a new ext hD where i am here, but that may not work, and I don't want to fork out another £50 if it's unnecessary.

Apologies for the long-winded explanation...+ many many thanks in advance....