error 21 cancelled installation of ubuntu. I have ample free space on drive c. How do I learn what the error is all about and how to rectify the problem

Do you known that installing other operating system it will delete your Windows? You need to have free partition. Do you have it, or do you really want to remove Windows?

Sorry, but with Ubuntu this doesn't have to be true. You can install Ubuntu alongside Windows in an image file on the Windows partition (Wubi-install).

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tto eselix i was aware that windows and linux would coexist in the allotted spaces. I'm wondering if I should have disconnected my external HD. I allowed linux to select the space on my HD by default

The Grub error 21 is Selected Disk Does not exist. For more information and details google grub manual and read.

You haven't posted enough information for anyone to be of specific help. Since you got to the point mentioned, you should boot the Ubuntu CD, click on the Applications tab shown when you get to the Desktop and open a terminal. In the terminal type: sudo fdisk -l(lower case Letter L in the command). This will output your drive/partition information which will be useful if you post it here.

Indicate which drive are you trying to install Ubuntu to?

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Well, I didn't known that, thanks.

(Not an answer; correction of comment)
You can install Ubuntu along with an existing Windows with the normal installer on the Ubuntu CD. Select "Install Ubuntu alongside Windows" and the installer will automatically re-partition the disk. As usual before re-partitioning disks, make a backup of valuable data on your Windows partition first.

See https://help.ubuntu.com/community/WindowsDualBoot and for nice screenshots see https://help.ubuntu.com/community/GraphicalInstall (except in step 5 "Allocate drive space" select "Install Ubuntu alongside Windows").

Thanks Yancek, I booted linux from bootable disk and allowed it to go ahead and instal Ubuntu. I'm still a learner. I have ample space on my HD but also had an external hard drive plugged in during the procedure so am unsure whether this contributed to error21.