imac won't play DVDs in Yosemite

sorry i'm so computer clueless! appreciate the help, would understand if you feel like giving up

No, not at all.

I'm just a bit loathe to advise you on something that I have no direct experience of. (Core Storage and TM).

If you open a new finder window and go right to the top level, (you can go up levels if you press and hold command and then press the up cursor repeatedly until you see no further changes - ⌘▲), it should end up with a window that has your boot drive, amongst other things. Highlight that, then press and hold command and then press the letter i - ⌘i. Screen shot the new window that appears.

BTW is that your only external drive?

Yes, my external hard drive is made by WD (WesternDigital)

That makes a little bit more sense. It more closely resembles the image you posted earlier, (where it says you have 19MB free), as it looks like you've used a about a fifth.

Actually, do you still have an older OS to load up at this point?

I've got plenty of time machine backups on my external hard drive from before i updated to yosemite from mavericks so can i extract it from there?

Yup.

I've been reading a little bit about Core Storage. I seems it's just the way the Mac sees the two separate volumes as one drive, (you do have a fusion drive right)? Hopefully someone with direct experience will validate this statement.

If that's the case then after you've done a backup we should be Ok to proceed.

i think i have a fusion drive? does this help

Not quite, what does System Profiler say?

 > About this Mac > Storage?

You're looking for something similar to the below;

Or....

 > About this Mac > System Report > SATA/SATA Express ?

No, you have no Fusion Drive.

Lex, have you any idea why the DU screen shot posted by Alidonostia is different from mine, I don't have that Logical Volume Group business?

Also, am I the only person this happens to;

ah this

or these

I answered a question of Alidonostia: no FusionDrive !

I do not understand your question,

Lex

Yup, I see that.

DU shows my internal hard drive, (from a clean Yose install), as not having a logical volume group. Have you any idea why this might be so?

Do you want to ask me whether you have a fusion drive (OSX) or not? Don't you know yourself?