Question1) How can I retain the captions when transferring the pics over to the hard drive?
I don't know if this is still relevant as it was written quite some time ago, but have a look:
Assigning iPhoto Keywords and Comments to EXIF Data Fields
(just pay attention to the part about moving comments to EXIF)
There may well be a program or script that can do this for you less painfully by now. I don't know.
Question2) After safely transferring the pics over, is it safe to delete the pics to make more room for "start-up disk"?
The "start-up" disk is your internal hard drive. It would probably be wiser (not to mention easier) to just move the entire iPhoto Library to the external drive. Here's how you do that:
1. COPY (not move, COPY) the entire iPhoto Library package (file) to the external drive.
2. Open iPhoto while holding down the Option key. This will produce a dialog box asking if you want to create a new iPhoto Library (you don't) or open one (yes). This leads to an open file dialog. In that dialog, navigate to the "new" iPhoto Library (the one you moved on the other drive) and simply select it.
3. Now iPhoto thinks the default library is on your external drive, and it is safe to delete the old one. CAUTION: do not open iPhoto unless the external drive is ON
and visibly mounted on your desktop. You may wish to (at some later point) create a smaller "on the go" library of pictures in the place where the "old" library was so you can access some pictures when you use the laptop away from home.
You should OF COURSE make a full, separate backup of the entire iPhoto Library to another drive BEFORE doing any of this. Just in case (and see below).
Question3) How can I check if "start up disk is full" mean computer memory is full?!
Two things:
1. Memory = RAM. Storage = hard drive space. A "start up disk is full" message means you are running out of STORAGE space, not memory.
2. Running out of storage is BAD and will cause BAD THINGS to happen (data loss) if you continue. To check how much storage space you have available, you can simply open any Finder window (say, for example by double-clicking on the hard drive) and the figure is at the bottom of the window.
There's no hard-and-fast rule of how much you should have, but if the figure you get back is anything less than 20GB you should be worried and start planning immediately to figure out a) what files you can offload elsewhere or just get rid of, OR b) where to buy a bigger hard drive. LIke RIGHT NOW.
First and foremost you really, REALLY,
REALLY need to make a backup. Get another external drive if need be, but you MUST do a backup or you ARE putting your data at serious risk. Get a drive that has more capacity ("is larger") than the boot drive.