My OSX is 10.6.8 and i want to upgrade to the last lion mountain version but i dont have any external disc drives, so i am not able to do a backup with the time machine. Is it very risky to do the upgrade without the time machine backup? Tanks,
Rule: Always back up at least your own document, photos etc — or your whole User Directory, if you've got the space - before any big changes.
You don't necessarily need to backup the OS — though a disk clone onto an external USB, made with SuperDuper or CarbonCopyCloner, is an instant rescue, right at hand, in the event of a crash or data loss, as well as another Startup Disk from which to repair your hard disk.
If you've got access to an OS install image, download or disks, in the event the upgrade doesn't go well and you need to downgrade, you don't need a clone (though it's easier) but just a copy of whatever you don't want to lose.
Related Rule: Always routinely back up / copy to somewhere else anything you don't want to lose.
You don't need Time Machine. It's just one method, one kind of external drive.
I use Arc, for example, which backs up what I tell it to (to very cheap storage using A3) every night, silently, from the menubar. I also clone every few weeks to a small USB external.
Another inexpensive way is to copy / backup to a flash stick - mine holds 16GB, which is enough for my work. A photographer or video person would of course would need much larger backup and storage.
It's a broad subject — but just don't mess with big changes "unprotected." Data loss happens.
Save your data to DropBox, that's one easy step.
Partition a section of your hard disk's free space, and copy the files you want to keep into there.
Upgrade your version of OS X to Mountain Lion and then once the upgrade is complete, check it out. If everything is OK, move the important files back into the main drive and extend out the partition to recover the lost space.
Second option is to install DropBox and ensure it keeps a backup of the files that you want. Or use Mega, because they give you more free space and it's encrypted (don't forget your password!), and do a clean install, then copy the files you want back down afterwards.