I have dubbed vcr tape to a Sony DVD-RW disc and tried playing it back through a toshiba player and it keeps coming up Bad Disk. My PC won't even recognise it. When started dubbing the screen displayed 1hr...
Did you say you used a DVD-RW disc to transfer a videotape onto? The reason your PC won't recognise that disc is because the data format on it is a different language to what PCs understand - plus an unfinalised DVD-RW won't play in other DVD players - it is usually an incompatible format.
(DVD+RW discs with titles/top menus created in video mode CAN be played on a PC DVD drive or a normal DVD player as can finalised DVD-R discs (DVD-R discs MUST be recorded and finalised in the same recorder for compatible playback). DVD-RW discs can only be viewed on the unit they were written/recorded on.
The other reason for the 'remaining space' on the disc you describe showing less than 120 minutes is due to a title recently being recorded or erased - unless you format it - the 120 minute / 4.7GB DVD-RW will not show full available space. Erasing titles on re-writable DVDs does not increase space. The same rule applies to unfinalised +R / -R discs (you cannot re-format these to get the space back once recorded - they are 'once only' writables)
I recommend you invest in some DVD-R discs and re-copy your VHS tape to that - then if you are satisfied with the result, title etc., finalise the DVD-R - it will play on any DVD player or PC with DVD support (or it should). These discs are relatively inexpensive to buy just about anywhere.
Final note: do NOT finalise the DVD-RW you recorded (I hope you haven't already done this) - simply format/erase it - re-use it for something else or keep it until you need it.
You can use any brand of DVD, just take note of the format before recording/playing.