The site collection could not be restored. Please make sure the content databases are available and have sufficient free space

When the restore process ends unexpectedly in middle and when you are trying to do the same restore again this is the error message you would be getting eventhough it had enough space in content DB. This is due to the Orphaned items that are created by previous restore process. We can get the list of orphaned objects by using the STSADM databaserepair command
Syntax:
stsadm -o databaserepair -url -databasename [-deletecorruption]

WEBAPP URL: URL of the web application
CONTETNDBNAME: Name of the content database
-deletecorruption: This parameter is optional. On giving this parameter it deletes Orphaned objects.

Getting List of Orphaned Objects:
stsadm -o databaserepair -url -databasename
Ex: stsadm -o databaserepair -url http://mywebappurl/ -databasename
Below is the screen capture of the list of orphaned objects

Deleting the Orphaned Objects:
stsadm -o databaserepair -url -databasename -deletecorruption
Ex: stsadm -o databaserepair -url http://mywebappurl/ -databasename -deletecorruption
Once this is entered it displays the Orphaned Object ID and the status of it(Deleted). Below is the screen capture for reference

After these two operations you should be able to run the stsadm restore operation. Hope this helps.

Cheers.

Vinay.