Does RAM error cause any aftereffect

This PC suddenly freeze and become unresponsive while it was transferring data to an external portable hard disk and needs to be forcefully shut down by pressing the power button. However, when it reboot it gives off the double beep sound continuously and wouldn't load into Windows. After searching the web, it seem likes a RAM problem. After trying a few time by removing and putting back the RAM and trying to turn it on a few more time, the PC finally load the windows and can be used.

Will this affect the PC and external hard disk data or performance like the anti virus software or cause any problem that need to be take note on?

Bad RAM can cause a wide variety of problems. You need to determine of this is the case and replace if necessary.

Generally, if hardware shown a problem it's likely that it will repeat, unless it was only a seating problem. If you successfully identified RAM as the offending component and it repeats again, I would seriously consider replacing the memory modules to prevent further problems in the long run, like more crashes and BSODs.

As for that particular crash, just perform a chkdsk on the system disk and the external HD you were copying to. Also, to be safe, delete the files being transfered and copy them again to prevent corrupt data. I wouldn't expect any other serious problem from a system crash.