I had no problems with the installer in my multi-boot environments (using grub and bootit NG) through version 3.5; no joy since then through 10.1 alpha2.
This applies to the i386 versions; haven't tried the 64 bit. Apparently something changed with the installer at version 4.x that introduced the error, which appears very early in the install process with identical error messages pointing to the installer's partitioning (either auto or manual option).
I tried to upload a typical pc-sysinstall.log, but got an error that .log and .txt files are not allowed for upload. So I'll just include the content of the file:
kern.geom.debugflags: 0 -> 16
`Running gpart modify on /dev/ada1
Running: gpart modify -t freebsd -i 1 /dev/ada1
Running: gpart create -s BSD /dev/ada1s1
gpart: geom 'ada1s1': File exists
EXITERROR: Error 1: gpart create -s BSD /dev/ada1s1
Running: umount -f /mnt
Unmounting DVD/USB media: /cdmnt-install
Running: umount -f /cdmnt-install