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
ada1s1 modified
Running: gpart create -s BSD /dev/ada1s1
gpart: geom 'ada1s1': File exists
EXITERROR: Error 1: gpart create -s BSD /dev/ada1s1
Unmounting: /mnt
Running: umount -f /mnt
Unmounting DVD/USB media: /cdmnt-install
Running: umount -f /cdmnt-install

I will like to know how you did your partition? The log are only use full if I have this information.

Install attempt was over last successful version 3.5 install that was cleared by (most recently) version 10.1a2 installer using auto partitioning option; original partitioning on drive by gparted.. Previously have tried minimal manual partitioning (root+swap) with same results (version 4.x installers)...